1. Field of the Invention
The present invention relates to a portable gas detection dosimeter carried by an individual to monitor the levels of airborne chemical contaminants to which the individual is exposed, and to provide an alarm when certain short-term or long-term weighted average exposure limits to the contaminant are exceeded.
2. Description of the Prior Art
In many industries workers are exposed to airborne concentrations of chemical substances such as carbon monoxide, sulfur dioxide, hydrogen sulfide, nitrogen oxide or chlorine. If the levels of these contaminants are sufficiently low, the worker may be repeatedly exposed to them day after day without adverse effect. However, if the concentration levels increase sufficiently, exposure to the contaminants may result in physical irritation or impairment of health of the individual.
Such adverse physiological reactions may occur either as a result of short term exposure to relatively high concentrations of the contaminant, or long-term exposure to relatively lower concentrations. Various governmental and other safety standards now are being promulgated to set acceptable limits for such short and long-term exposure to various airborne chemical substances.
One such standard is the "short-term exposure limit" (STEL). This is the maximum concentration to which a worker can be exposed for a period up to fifteen minutes continuously without suffering from irritation, chronic or irreversible tissue change, or narcosis of sufficient degree to increase accident proneness, impair self-rescue, or materially reduce work efficiency. Another standard is the "time-weighted average" (TWA) concentration for a normal eight hour work day or forty-hour work week, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect. Thus the STEL and TWA represent two categories of "threshold limit values" (TLV) which represent maximum allowable exposure limits to the airborne contaminants.
In the past, such contaminant levels have been measured by using an electrochemical monitor placed in the work area. This approach, however, often did not provide an accurate indication of the actual exposure of an individual worker. Since the instrument only measured the contaminant concentration at a single location, it did not indicate the actual levels to which the worker was exposed as he moved from place to place in the course of his work. Thus, e.g., the time weighted average concentration level at the fixed location of the monitor may have been within an acceptable limit, whereas the average contaminant level to which a particular worker was exposed during the work day may have exceeded the acceptable limit, as a result of his periodic incursions into areas of higher concentration.
To alleviate this shortcoming, a principal object of the present invention is to provide a small, portable dosimeter which is carried by the individual worker so as to monitor the actual airborne contaminant levels to which the individual is exposed. Should the short-term or TWA threshold limit values be exceeded, the dosimeter provides an audio and/or visual alarm to warn and notify the individual of the excess exposure condition.
Electrochemical sensing cells for detecting airborne contaminant chemicals are known per se. For example, such a cell is shown in inventors' U.S. Pat. No. 4,017,313. This device consists of a sensing electrode and a counterelectrode, in contact with an electrolyte. When the contaminant gas contacts the sensing electrode, either electrooxidation or electroreduction occurs, depending on the species to be detected. A complementary reaction occurs at the counterelectrode. These reactions cause an electrical current to flow, the magnitude of which is proportional to the contaminant concentration. By appropriate selection of counterelectrode and electrolyte materials, and by using a bias voltage of appropriate potential, the sensing cell may be made selective to a particular gas species.
A shortcoming of prior art electrochemical sensing cells is their relatively large size, which prohibits incorporation into a small, personnel-carried instrument. Thus another object of the present invention is to provide an electrochemical sensing cell having a configuration and small size which permit its incorporation into a portable dosimeter intended for individual use. The electrochemistry of such cell is well known, and indeed is the same as that employed in known cells such as that shown in the inventors.degree. U.S. Pat. No. 4,017,373. However, the cell configurations, including the assemblies utilized to bring air samples into contact with the sensing electrodes, are new and well adapted for use in a dosimeter.
Prior art electrochemical sensing cells of the type described above provide an output current which is proportional to the concentration level present at the cell. That is, the cell itself does not provide an indication of the long-term average concentration level. Rather, this must be obtained by appropriate averaging or integrating circuitry, such as that provided in the dosimeter disclosed herein.
One prior art means for achieving such averaging or integration is by the use of a coulometric device called an "E-cell," which is connected to the output of the electrochemical sensing cell. This silver-nickel device initially is charged like a battery, to a certain charge level. The cell is then exposed to the air, for example, during an eight hour work shift. At the end of this exposure time, the E-cell is discharged at a fixed rate. The amount of time that the E-cell takes to discharge to a reference level is indicative of the time weighted average concentration of the contaminant over the exposure period.
Several disadvantages are inherent in the use of such an E-cell. During the work shift, no indication is provided by the E-cell of the present exposure level. Thus if the worker should be exposed momentarily to a contaminant concentration level which is above the short-term exposure limit, the E-cell will provide no indication of this, and no warning will be given to the worker. Moreover, the time weighted average exposure level can only be obtained at the end of the day when the E-cell is discharged. Thus if the worker is exposed to contaminant levels which exceed the acceptable TWA in less than the full work shift, no indication of this adverse condition will be provided.
By contrast, an object of the present invention is to provide a dosimeter which does not use a coulometric integrating device, but which incorporates appropriate circuitry for use in conjunction with an electrochemical sensing cell to provide an immediate warning to a worker when either the short-term exposure level or the long-term time weighted average acceptable exposure level has been exceeded.
Another object of the present invention is to provide a personnel-carried dosimeter which will maintain a record of the actual contaminant exposure levels to which the worker was exposed during the day. The dosimeter then can be connected to an external readout device, such as a printer, to obtain a record of this contaminant exposure history.